When do real gases behave most like ideal gases?
Under low temperatures and high pressures, gases behave less like ideal gases and more like real gases.
An ideal gas has the following qualities:
- The particles are extremely tiny compared to the volume filled by the gas – like mathematical points.
- The particles are very far apart and moving fast.
- The particles have no features (shape) and exert no forces on each other other than by collision.
This is approximated well by most gases when they are at low pressure, so that the particles are small and far apart compared to the volume of the gas, and when the temperature is high so that the particles are moving fast.
When the pressure is increased, the number of particles in the same volume increases, so the volume of the particles becomes a bigger proportion of the volume of the gas. When the pressure is high enough, we can no longer assume that qualities 1 and 2 above are the case. When the temperature decreases the particles move more slowly, which makes the part of quality 2 about moving fast invalid, and as they move slower they are more likely to exert electrical forces on each other, making quality 3 invalid.